9.2 The base population for population estimates is derived from the latest available Census of Population and Housing and comprises Census counts by place of usual residence.

9.3 Since 1961, a Census of Population and Housing has been conducted every five years. The objective of the Census is to measure accurately the number and key characteristics of people in Australia on Census Night, and the dwelling in which they live.

Scope and coverage of the Census

9.4 The Census aims to count all people actually located in Australia on Census Night. This includes Australian residents in Antarctica and people in the territories of Jervis Bay, Cocos (Keeling) Islands and Christmas Island. The other Australian External Territories (Norfolk Island and minor islands such as Heard and McDonald Islands) are outside the scope of the Census. The only group of people who spend Census Night in Australia but are excluded from the Census are foreign diplomatic personnel and their families.

9.5 The Census includes people on vessels in or between Australian ports, on board long-distance trains, buses or aircraft and on oil or gas rigs off the Australian coast. People entering Australia before midnight on Census Night are counted while people leaving an Australian port for an overseas destination before midnight on Census Night are not. Visitors to Australia are included regardless of how long they have been in the country or how long they plan to stay.

Census counts by place of usual residence

9.6 Census counts are available by either place of enumeration (sometimes referred to as 'de facto counts') or place of usual residence (sometimes referred to as 'de jure' counts), based on responses to the Census question on address (see paragraph 2.9). For the purposes of producing population estimates, counts by place of usual residence are used.

9.7 Usual residence is defined in the Census as the place at which a person has lived or intends to live for six months or more. While for most people their usual residence will be the same as their actual location on Census Night, some people will spend Census Night at a place other than their usual residence. This means that their place of enumeration will differ from their place of usual residence.

9.8 People visiting Australia on Census Night (i.e. those who usually reside overseas) are included in Census counts by place of enumeration, but are not included in counts by place of usual residence.

Non-response to Census questions

9.9 Besides the question on address, questions on age, sex, marital status, country of birth and Indigenous origin are particularly important in the production of population estimates. Instances in which people do not answer these questions therefore have a bearing on the accuracy of Census counts, although information is imputed by the ABS from other details supplied on the Census form or, failing that, information provided by the Census collector. Details are imputed for all instances in which there is no response to questions on age, sex, marital status and usual address.

9.10 Table 9.1 shows that non-response to the questions which are of importance to population estimates has increased in recent censuses.

9.1 Rate of non-response to Census questions - 1991 to 2006

Census

1991

1996

2001

2006

Variable

percent

percent

percent

percent

Sex(a)

na

na

na

na

Age

2.0

2.2

3.6

(b)5.0

Usual residence

5.8

0.2

0.2

4.1

Marital status(c)

6.8

2.0

2.7

5.0

Country of birth

2.2

3.5

5.5

6.9

Indigenous origin

3.2

3.0

4.1

5.7

na not available

(a) Not available but believed to be insignificant.

(b) In 2006, respondents were given the choice of providing their date of birth or age last birthday. These were combined to calculate their age on Census Night.

(c) As a per cent of the population of all ages. However a considerable proportion of non-responding persons were aged under 15 years and never married. All persons aged under 15 were coded to never married.

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